Stop creating duplicate enum members when the element model has already been built

pkg/analyzer/lib/dart/ast/ast.dart

Index: pkg/analyzer/lib/dart/ast/ast.dart
diff --git a/pkg/analyzer/lib/dart/ast/ast.dart b/pkg/analyzer/lib/dart/ast/ast.dart
index 035b7f58be5c6f4d7f037cea6dae61d946ffcd5e..1237fd1d7148a7b6325d6e7197886b2da6013b4a 100644
--- a/pkg/analyzer/lib/dart/ast/ast.dart
+++ b/pkg/analyzer/lib/dart/ast/ast.dart
@@ -2856,6 +2856,9 @@ abstract class EnumDeclaration extends NamedCompilationUnitMember {
    */
   NodeList<EnumConstantDeclaration> get constants;
 
+  @override
+  ClassElement get element;
+
   /**
    * Return the 'enum' keyword.
    */